The rotating metal heat recuperators of prior art, such as the highly successful Ljunsgtom type, are subject to flue dust erosion and acid mist corrosion below about 300.degree. F. Likewise their enormous fabricated metal weight and structural support are expensive and support bearings and labyrinth gas seals are complex. While the straight-through-downward path of flue gases between closely-spaced metal sheets provides self cleaning features, it does not provide the enormous heat transfer rates under turbulent flow around each particle afforded by rock beds. The enormous size of subterranean rock beds disclosed in my copending patent application, Ser. No. 639,307, filed Aug. 9, 1984, now U.S. Pat. No. 4,597,433, granted July 1, 1986 are avoided by merely rotating the beds, while the dampers to reverse direction of gas flows are eliminated.
No prior art is known which in one apparatus continuously first regenerates the sensible flue gas heat from the range of 800.degree. F. to 250.degree. F., second recovers the heat of condensation from 250.degree.-110.degree. F. and lastly regenerates the sensible heat from 250.degree.-70.degree. F. which is necessary to insure sulphur dioxide fumes are dissolved in the water of condensation.